1. Field of the Invention
The present invention relates to a method and apparatus for selectably positioning a head over data storage tracks on a rotatable disc in a disc data store.
2. The Prior Art
It is well known to employ a feedback servomechanism to position a magnetic read/write head over selectable, concentric circular radially equispaced data storage tracks on a magnetic rotatable disc in a disc data store. It is also well known to employ as the positional feedback device in such a servomechanism, a plural phase transducer providing as position indicating output a plurality of cyclically distance repetitive signals. Magnetic and optical grating devices are in common use, providing triangular or sinusoidal outputs.
Mechanical tolerances, thermal dimensional changes, and transducer inaccuracies are reflected in a misplacing of the head over the track. Servo tracks may be recorded among the data tracks for purposes of referencing all positions to the disc. As the amount of data on each disc increases, and the tracks become closer together, this measure alone is insufficient for the accurate placing of the head over a track. The common usage of plural phase position transducers has, inherently therein, an error resulting from inaccuracies in the relative phase between signals. This error becomes significant at high track densities.